Today, photolithography requires short exposure wavelengths for successful imaging of very small semiconductor device dimensions on a wafer. At wavelengths in or below the deep ultraviolet (DUV) range, e.g., below approximately 300 nanometers, the materials used to produce a photomask assembly have become increasingly important. At these shorter wavelengths, the light in a photolithography system has more energy and can be destructive to a photomask assembly manufactured with conventional materials.
A conventional photomask assembly generally includes a photomask, also known as a reticle or mask, and at least one pellicle assembly that covers a patterned side of the photomask. A standard photomask includes a patterned layer of opaque or partially transmissive material formed on a transparent substrate. A pellicle assembly typically includes a thin film attached to a frame. The thin film acts as a cover that keeps contaminants off an adjacent surface of the photomask during a lithography process. The pellicle frame is typically mounted on the photomask with an annular shaped adhesive gasket that is attached to the bottom and around the perimeter of the pellicle frame.
At DUV wavelengths and below, flatness of the photomask is a concern. Existing pellicle mounting techniques often use pressure to create a seal between the pellicle frame, the gasket and the transparent substrate. Before the pellicle is mounted on the transparent substrate, the pellicle frame may contain up to approximately one hundred microns of warp. In contrast, the transparent substrate typically contains less than approximately two microns of warp before pellicle mounting. Since the pellicle frame warp frequently exceeds the mask warp, voids may be created between the gasket, the pellicle assembly and the photomask.
In order to eliminate the voids and create a more complete seal, a pellicle pressing tool is sometimes used. Typically, a force of twenty-five to fifty pounds may be applied to the pellicle frame, which yields a pressure at the interface of the frame, the gasket and the photomask of approximately fifteen to thirty pounds per square inch (psi). This force compresses the adhesive gasket and presses the frame against the mask to complete the seal.
During mounting, any warp in the pellicle frame may be overcome to completely seal the gasket to the frame and the photomask. The frame attempts to return to its original shape when the pressure is removed, which causes photomask distortion. The photomask assembly eventually reaches an equilibrium, but the frame and photomask remain in a stressed state. Over time, this stress causes the flatness of the photomask to degrade and can cause registration errors on a wafer during a semiconductor manufacturing process.